The study's objective is to determine whether atracurium or vecuronium is better for muscle relaxation in critically ill children who require mechanical ventilation. The end points of the study are the number of dose adjustments for each agent during the course of therapy and the time to return of normal neuromuscular transmission after the agents are discontinued. Eleven patients have been enrolled in the study to date. Since the study is blinded, no results are available. Although neuromuscular blocking agents have been used as adjuncts in critically ill patients on mechanical ventilation for the past 30 years, a controlled trial has never evaluated their use. This study is the first scientific trial evaluating the dosing requirements and recovery times of these agents in critically ill patients. The protocol was terminated on August 1, 1996. Eleven patients were enrolled in the protocol. The gender and racial distribution was four male and seven female, four Caucasian and seven African American. Two patients died from their underlying disease while they were enrolled in the protocol. No side effects were attributed to the protocol drugs.